The invention relates to hermetic packages or hermetic substrates for electronic devices. The invention also relates to methods and apparatus for manufacturing such hermetic packages or substrates.
Many electronic devices, such as semiconductor integrated circuits, must be packaged in clean inert atmospheres for obtaining reliable operation over long lifetimes. In addition to being hermetic, such packages must provide electrical connections between the circuits inside the package and external devices outside of the package. Often, the package also provides electrical connections between different points on the circuit or circuits inside the package. Such a package may comprise, for example, a hermetic substrate sealed to a hermetic lid by way of a flange so as to form an enclosed chamber containing one or more electronic devices.
One type of known electronic device package (see, for example, U.S. Pat. No. 4,234,367) consists of a multilayer glass ceramic substrate. On one side, the substrate has termination pads for attaching electronic devices. On the other side, the substrate has termination pads for making external connections.
Each layer of glass ceramic may be provided with one or more electrically conductive thick film lines on the surfaces of the layer, and one or more electrically conductive vias passing through the layer. The vias connect electrically conductive thick film lines and/or termination pads on opposite surfaces of the layer. Vias which extend to termination pads on the substrate surface must be accurately positioned at locations corresponding to the locations of terminals on the electronic devices to be packaged.
Such multilayer glass ceramic substrates are manufactured by producing a slurry of glass particles in a binder. The slurry is cast and dried into green sheets. Via holes are punched through the green sheets in desired configurations, and a copper paste is extruded into the via holes. A copper paste is also screen printed onto the green sheets in a desired conductor pattern to form line interconnections and voltage planes. A plurality of sheets are laminated by pressing above the glass transition temperature of the green sheets (typically 70.degree.-100.degree. C). Finally, the laminated sheets are sintered.
After sintering, the substrates may exhibit certain structural irregularities that may adversely affect the hermeticity of the substrate. Consequently, it has been known to "back fill" the gaps and cracks in the substrate with a polymer or other sealing material after completion of sintering, in order to obtain a hermetic substrate. Such an additional processing step is, however, costly and time consuming.
After the substrate is "back filled", the opposite surfaces of the substrate must be ground and polished flat and parallel to each other to provide suitable mounting surfaces for electronic components and external connections. It is important that the conductive vias be nondistorted so that termination pads are properly aligned with electronic components and external connections to be mounted on the substrate.